Developing baths and fixing baths are used in large quantities for the development of black and white photographs. Nowadays developing baths preponderantly contain sodium sulphite as the oxygen acceptor, together with hydroquinone, potassium bromide, carbonates, borates or phosphates and sodium hydroxide as the basic component. Fixing baths contain as the main component thiosulphates generally in the form of up to 90% ammonium sulphate and 10% sodium thiosulphate, as well as sulphites and bromides and optionally potassium alum and usually acetic acid as the acid component.
Apart from unusable excesses of the aforementioned components, spent baths contain complex-bound silver ions, water-soluble constituents from the photographic material, such as colloids from emulsion and protective layers, platicizers, etc. According to modern environmental protection regulations these waste baths constitute hazardous waste, which must be disposed of or processed in accordance with the legal requirements, so as to be dischargeable into the standard waste cycle.
It has long been known to recover the most valuable component of spent baths, namely the silver contained therein. This take place by the electrodeposition of the metal or by precipitation using hydrogen peroxide. The desilvered baths can be further processed by oxidation and precipitation reactions (DE 33 37 215 A1) in that fixing and developing solution are mixed and hydrogen peroxide added. The sulphides, sulphites and thiosulphates are transformed into alkali sulphates and then milk of lime is used as the precipitant. As a result large calcium sulphate (gypsum) quantities are obtained. Even larger gypsum quantities are now obtained during purification and cleaning processes for acid media (flue gases, waste water, etc.) by adding milk of lime. These large process gypsum quantities are scarcely economically usable, because the degree of purity is unsatisfactory for many applications.
It is also known to distil in a vacuum the spent, desilvered baths. Mainly the salt concentration is raised and the hazardous waste volume reduced by approximately 20 to 30%. The water condensate obtained still contains too many impurities from the photographic baths to allow introduction into the waste water system. The further purification using ion exchangers or reverse osmosis is complicated and therefore uneconomic.
The combustion of spent photographic chemicals by feeding into waste incineration plants leads to a rise in the SO.sub.2 content of the flue gases and therefore to an additional burden of the flue gas desulphurization plant, as well as the gypsum budget.
The electrodialytic regeneration of photographic baths (DE 30 22 305 A1, DE 30 22 328 A1) leads to the recovery of silver ions. In addition, salts are obtained, which have not been used in practice in the preparation of new baths. In addition, the constituents of the thus processed solution must be supplemented.
It is finally known (DE 34 40 894 A1) to process photographic chemicals with considerable technical effort, particularly the use of energy and chemicals, leading to distilled water, an approximately 10% ammonia solution, calcium sulphate and ammonium/sodium thiosulphate. There is an approximately 50% thiosulphate recovery, but cannot be used again in photographic baths due to its impurities. The recovered ammonia solution quantities are also economically insignificant. Once again large quantities of gypsum and contaminated water condensate are produced.
The problem of the invention is to process the developing and fixing baths used in black and white processes in a single, multistage process to novel substances in an environmentally compatible manner.